That is a hard question to answer (10,20 or 30%). If you are looking for a gold ring, why use the prospecting mode when the coin/jewelry mode will give you more information. Do you have a comparable size gold ring to make a comparison? Is it a small ladies wedding band or a large man ring? What is the Karat of the lost ring?
All of these factors come into play!
If you find an item in the prospecting mode at 10 or 20%, which is where I would suspect a gold ring to fall, switch over to the coin/jewelry mode and see what extra information you get to make a better decision.
It is hard to compare a nugget to a ring. But the smaller the nugget the more it falls into the iron range and therefore the reason I dig everything below 70%. Tip: you should know most gold rings fall into the foil/pull tab range on most detectors, that is why a lot are missed, for people do not like to dig foil and pull tabs.
Good Luck!